Watery eyes can be completely mystifying. You might expect to tear up when remembering a heart-pulverizing breakup or reading about unlikely animal friendships. But in the middle of the day while just going about your life? Not so much. Unfortunately there are plenty of reasons your eyes might suddenly spring a leak, even seemingly at random.

Before diving in, here’s a primer on why your eyes are wet in the first place.

Your eyes produce a fluid called basal tears in order to stay properly moisturized, which helps you avoid discomfort and see properly, according to the National Eye Institute (NEI). Basal tears are made up of three layers to form a mixture of fatty oils, water, mucus, and over 1,500 proteins so your eyes can do their job. When you blink, your basal tears spread across your corneas, the dome-shaped surfaces of your eyes that protect against bacteria and help focus light so you can see.

“Your corneas have to have this really thin layer of tears all the time or you can’t see clearly—if you’ve ever held your eye open for too long, you know how blurry it gets,” Joel Hunter, M.D., founder of Hunter Vision, tells SELF.

Excessively watery eyes have many potential causes, but they usually happen thanks to one of two mechanisms.

Your eyes typically have a system to ensure they’re not too dry and not too wet. The lacrimal (tear) glands underneath your eyelids supply a fluid mostly made of salt and water, according to the Cleveland Clinic. This spreads across your eyes as you blink, then any excess tear fluid typically drains through four tear ducts known as puncta, which are at the inner corners of your eyelids near your nose. Sometimes, though, that system can hit a snag.

“There are two very broad categories ophthalmologists use to talk about excessive tearing: overproduction of tears and insufficient drainage,” Lora Glass, M.D., assistant professor of ophthalmology and director of Medical Student Education in ophthalmology at Columbia University Medical Center, tells SELF. Either one of these can lead to tears flowing down your cheeks, no matter the cause.

Here are some of the more unexpected reasons your cup eyes might runneth over.

1. It’s so dry outside/inside.

A dry environment doesn’t just sap your skin of moisture; it can also dry out your eyes. When this happens, your lacrimal glands will produce and release more tears in what’s known as reflex tearing, Dr. Glass says. (This is also why your eyes tear up when it’s windy outside.)

If you think this is your issue, the Mayo Clinic recommends lifestyle changes like getting a humidifier, avoiding things like air conditioners and fans blowing into your eyes, and using artificial tears. (Dr. Glass suggests avoiding eye drops that promise to reduce redness, since they often have blood vessel-constricting agents that can make redness worse when used consistently over time

2. You’ve been laser-focused on your computer screen.

Zoning out at your laptop can make you blink less often, according to the Mayo Clinic. This can lead to dry eyes, then reflex tearing, Dr. Hunter says.

To combat this, try the 20-20-20 rule: Every 20 minutes, focus on something 20 feet away for 20 seconds. “It lets your eye rest and get to blink a little,” says Dr. Hunter. You should also make sure your computer screen isn’t above eye level, because then you’ll naturally open your eyes wider so you can see, says the Mayo Clinic.

3. Your contacts are drying out your eyes.

“At the end of the day, [contact lenses] are pieces of plastic sitting on the front of your eyes, so they can be disruptive to tear film,” Dr. Hunter says, explaining that your eyes might overproduce tears as a response.

But if you consistently experience watery eyes when wearing contacts, it could be a sign that you’re wearing them for too long or that they’re not the right fit, Dr. Glass says. If you’re worried about how your contacts are affecting your eyes, see your ophthalmologist for guidance.

4. Your makeup is getting in your eyes (sometimes without you even realizing it).

This is especially common if you practice tightlining. “Some people put eyeliner on that margin between the eyelashes and the eyes, which is where the Meibomian glands open up and release a thin layer of oil,” Dr. Hunter says. “All day long, that layer of oil wicks onto the front of your eyes to keep tears from evaporating too quickly.” If this swipes eyeliner onto your eyeballs, you might experience irritation, then tearing as your eyes try to flush out the makeup.

“Look at eyelashes as the sacred line you don’t cross,” Dr. Hunter says. (Though, if you have eyeballs of steel that never seem to be bothered by makeup, tightlining every once in a while may be OK, he says—just know that irritation is always a possibility.)

5. Or something else is blocking your Meibomian glands.

Anything that doesn’t let these glands release enough oil can cause overflowing tears, the Cleveland Clinic says. With that said, blocked Meibomian glands are more common in people with inflammation along the edge of their eyelids (blepharitis), rosacea, which can cause inflammation around the eyes, or other skin disorders, says the Mayo Clinic.

6. You’re having a reaction to something in the air.

When you come into contact with an allergen, your body releases histamine, a chemical that can cause the symptoms of an allergic reaction, like your eyes welling up. While you might expect this to happen in reaction to your known allergens, sometimes something more subtle can fly under the radar and set this off.

“Your eyes are sensitive organs out in the air all day long, coming into contact with many different kinds of substances,” Dr. Hunter says. If have random bouts of watery eyes and can’t figure out the cause, this might be why.

7. You have chronic dry eye.

Yes, it seems bonkers and hypocritical that your eyes can pingpong between being chronically dry and excessively watery. But that reflex tearing can come into play here, too. While anyone can experience symptoms of dry eye here and there, it can also be a chronic problem that makes your eyes overproduce tears in response, in addition to issues like stinging, burning, mucus in or around your eyes, sensitivity to light, blurred vision and eye fatigue, and more, according to the Mayo Clinic.

If your dry eye symptoms are persistent, talk to your doctor to see if they can determine the underlying cause, then a treatment plan that makes sense from there.

Sleep apnea (particularly the obstructive variety, which happens when your airway collapses or is blocked) also happens to be associated with a condition known as floppy eyelid syndrome, which is when your upper eyelids are more elastic than usual. Instead of staying closed while you sleep, this can cause your eyelids flop open, drying out your eyes in a way that can lead to reflex tearing, Dr. Glass says.

Sleep apnea is treatable through lifestyle changes, various devices, or surgeries in the most extreme cases—if you suspect you have it, see your doctor for testing.

If you have no idea why your eyes are tearing up and it’s lasting more than a few days, see your doctor.

Your eyes are powerful, but they’re also delicate. Don’t wait around if you think something’s wrong. If the tearing won’t go away, is happening with a significant amount of pain, if you can’t see as well, if light is bothering you, or if it’s coinciding with any other strange ocular changes or symptoms, Dr. Glass recommends seeing your doctor ASAP: “These are great reasons to see somebody urgently as opposed to waiting.”

While excessively watery eyes could be due to one of the above reasons, sometimes they can be one of many symptoms pointing to something even more unexpected, like a thyroid issue. Only a doctor can tell you for sure.